In typical thermal printing mechanisms, a plurality of electrically resistive elements is disposed on an electrically non-conductive substrate. The elements generate heat when an electrical current passes through them by the Joule Heating Effect. This heat energy is used to create dye images using thermally sensitive materials. Each resistive element forms an image pixel using such thermally sensitive or dye donor materials. As shown in FIG. 4, each resistive element has a rectangular shape and is directly connected to a power bus. In one application of the heads, the thermal energy is used to trigger leucocytic dyes in coated paper. This type of media changes density at a rate proportional to applied heat energy. In another application, the energy is used to transfer dye from a thin dye donor web to a dye receiving surface.
In these printing processes, the thermally sensitive media is fed through a nip formed by the head and a rotating platen. The head is pressed firmly against the media. As the drum rotates, the energy to the head is controlled relative to the turning of the drum such that the resistive elements form image pixels of dye on the receiving surface. For black and white text imaging, the pixels are on for sufficient time to ensure complete density change. Further control over the released energy allows for the development of density variation within the image pixel. If the density levels become fine enough, then a virtually continuous tone image can be formed. In color applications, successive patches of color are deposited on to the dye receiving surface to form images of varying hue.
In applications where the image is high resolution (sometimes greater than 1000 pixels in each line), controlling each resistive element directly can create excessive numbers of wire from the head to the controlling mechanism (see FIG. 4). In these applications, the data is transmitted to the head as a serial data stream. In addition, only a predetermined number or set of the resistive elements on the head are energized at any one time so as to reduce the current demand and heat load at the resistive elements. Each of these sets of mutually activatable resistive elements is known as a "group." The heads in these applications contain a series of semiconductor chips that decode the serial data into an on/off signal for each of a set of resistive elements. Typically, several chips are wired together on the head so as to form a group.
The electrical chips used on the heads consist of a serial shift register, a set of latches to capture the data in each of the shift positions, and a semiconductor switching element to energize each resistive element. Typically, a plurality of these chips are mounted on the head. Electrical connection are then made between the connector, the chips and the resistive elements. Currently, each chip typically controls 64 to 128 resistive elements, depending on current load. In a large image application, such as FAX machines with 1728 elements, 27 chips may be needed. Wiring every four chips together produces seven group enables.
Another method that is used to reduce the wire count from the head is to use diodes to matrix decode the data coming into the head. In these heads, the elements are again divided into groups. Each group shares a set of data lines. Diode logic is used to allow the energization of one group at a time. For example, in a 512 element head, every 32 elements could have common data lines, creating 16 groups. The connections from the head would consist of the 32 parallel data lines, the 16 group lines plus an addition six lines, creating about 54 connections. The wire count goes to uncomfortably high levels above 512 elements.
The heating area of the head is built up in layers of materials so as to optimize head performance. The base substrate properties may have several coatings to change the thermal conductivity of the head or to create a raised bead on the head. These layers will typically improve printing performance. After the resistive element is deposited, a very hard anti-abrasion coating is applied to protect the resistors from oxidation and abrasion by the media.
The resistive elements are created using one of two different processes. The first process is a thin film resistor, fabricated by a sputtering or vapor deposition process. Typically, these films are low-resistance, and higher resistances are achieved by using a folded, elongated path resembling a coiled wire to achieve higher resistances. The second resistor manufacturing process, labelled "thick film," screens a slurry of resistive material over the traces. The material is deposited as a line or series of pellets over conductive traces. The processes for creating the resistive elements has steadily improved with the goal of reducing the resistance variations between elements. Recent improvement in the shape and location of the resistive elements has permitted manufacturers to increase the image pixel pitch from 6 and 8 pixels per millimeter to 12 and 16 dots per millimeter.